Process for doping materials



United States Patent 3,141,849 PROtIESS FQR DOPING MATERIALS Eduard Erik, Julius Nicki, and Heinz Siibernagi, Burghausen, Upper Bavaria, Germany, assignors to Wacirer- Chemie Gmhlil, Munich, Germany N0 Drawing. Filed June 29, 1961, Ser. No. 120,513

Claims priority, application Germany July 4, 196%) 1 Claim. (Cl. 25262.3)

The present invention relates to an improved process for incorporating doping substances in materials, such as, elements, intermetallic compounds and alloys.

It is, for example, known that silicon can be doped by melting a boron containing glass filament lengthwise on a silicon rod and then zone melting the thus prepared silicon rod. The boron contained in the boron containing glass is thereby incorporated in the silicon and bestows p-conductivity to the rod and depending upon the boron content a corresponding specific electric resistance.

This mode of operation has the disadvantage that in addition to the doping substances other parasitic substances may be incorporated in the materials to be doped, such as, for example, alkali metals and oxygen. In addition, there is the danger that the surface of the silicon becomes slagged and that the crystal growth is hindered and disturbed thereby.

It is an object of the present invention to provide a process for doping material with the aid of a doping body while avoiding introduction of parasitic substances in the material to be doped. According to the invention the doping substance is contained in a doping body of the same material or of one or more of the substances in elemental form of which the material is to be composed after the doping operation and which does not contain any substances which are not intentionally present in the doped body after the doping.

The term doping body as employed herein signifies a body containing the doping substances and with which the doping substances can be supplied to the body to be doped.

The process according to the invention can be used in zone melting in crucibles or boats and in crucibleless zone melting. For example, in a zone melting procedure carried out in boat shaped crucibles a germanium rod can be placed in the bottom thereof and a doping body in the form of a germanium rod of the same length as the first germanium rod but of smaller cross-section and containing aluminum, boron or phosphorus as the doping substance placed thereover and the two rods then melted together zone wise.

In crucibleless zone melting a 300 mm. long 20 mm. thick silicon rod is associated as closely as possible with a parallel silicon rod of the same length but 3 mm. thick and containing boron or phosphorus as doping substance and the two thus associated rods subjected to crucibleless zone melting in which the travelling molten zone encompasses the entire cross-section of both rods. Such zone melting can be repeated several times. The resulting rod has a specific resistance substantially uniformly distributed over its entire length whose mean values does not deviate more than 1%.

A prerequisite for the uniform distribution of the specific electric resistance in the doped body produced is that the doping substance also be uniformly distributed in the doping body. If the doping body has a certain resistance profile or pattern, this is repeated in the body doped therewith but the resistance is higher as the content of doping substance is distributed over the larger cross-section of the body to be doped. It is possible therefore, by providing a certain doping substance distribution in the doping body to produce a predictable distribution in the body doped therewith.

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The attachment of the doping body to the body to be doped prior to the zone melting can be accomplished by known measures. For example, in vertical zone melting under vacuum or protective gas the doping rod can be tied to the rod to be doped at their upper ends with a stainless steel or tungsten wire. Also, simultaneous clamping of both rods in the upper holder is possible. It is furthermore possible to spot Weld the doping rod along the length of the rod to be doped by electron beams or focused ion or heat rays. The same results can also be achieved using high frequency electric heating which easily melts the surface of the rod to be doped and the doping rod. Furthermore, the doping body can be applied in the form of a weld bead by striking an are between the body to be doped and the doping body. The welding can also be accomplished with hot gases.

The process according to the invention offers the decided advantage that only by attaching one or more doping bodies on a material to be doped and zone melting such combination predetermined electrical, mechanical, optical, magnetic or thermal properties can be provided in such material. It is, for example, possible to reduce the resistance of a material to A by attaching two doping bodies instead of one or to /3 by attaching three doping bodies.

It is not necessary that the doping body be distributed over the entire length of the rod to be doped. It can, for example, extend over one part or several parts thereof. For example, if a rod is desired in which a certain section in the center thereof is to be provided with a certain property, a doping body of corresponding length is attached over such section. By attaching doping bodies containing oppositely active doping substances along the length of the body to be doped it is possible to provide differently doped zones in such body. For example, if a doping rod containing boron is attached to the first half of a silicon rod and a phosphorus containing doping rod is attached to the second half a rod is produced in which /2 is p-conductive and the other /2 is n-conductive.

The doping bodies employed according to the invention usually are in the shape of rods but the process is not limited to the use of definitely shaped doping bodies. The diameter of the doping body preferably is smaller than that of the body to be doped. It also is possible to subject equally sized rods to zone melting. Suited forms of the doping bodies, for example, are: filaments, rods, tubes, bands or individual relatively small rods, fragments, grains or definitely shaped platelets, spheres or cubes.

The position of the doping body on the starting body which is to be doped can vary and depends upon what form and what mechanical properties the doping bodies possess. In the simplest case a doping body of the same length and general shape as the starting body is associated parallel to the longitudinal axis of such starting body. However, it is also possible to wind the doping body spirally around the starting body. Relatively small doping bodies, such as platelets, spheres, cubes, grains or fragments, are preferably placed in a row on the starting body and attached thereto by one of the above-mentioned procedures. With silicon it is, for example, possible to obtain very exact resistance values by attaching equally shaped small doped platelets 2 mm. in diameter and 1 mm. thick on the silicon starting material and then subjecting it to zone melting. Spheres, cubes or angular platelets of the same size could be used instead of the round platelets.

If the definitely shaped small doping bodies are placed in a row with equal spacing on a rod or tube shaped starting body, a doped product is obtained in which the values of the desired property follow a straight lined course over the length covered by the row of doping bodies. Every non-uniformity in the arrangement of the doping bodies produces a fluctuation in the properties produced and it is possible thereby, for example, to dope different portions of the starting material more strongly or weakly. The same effect, of course, can be obtained with doping bodies of the same size which are differently doped or with uniformly doped bodies of different sizes even with uniform spacing of such doping bodies.

It also is possible to apply the doping body on the body to be doped by vapor deposit, for example, by treating the starting bodies at subatmospheric pressures with a gas stream containing the doping substances to deposit the doping substances as a firmly adhering layer upon such starting bodies. This procedure is illustrated in the following:

A gallium arsenide rod was arranged horizontally in a vessel and the lower side thereof covered by two metal sheets so as to have only a strip 2 mm. wide parallel to the axis of the rod uncovered. Zinc was evaporated in a molybdenum boat situated below the gallium arsenide rod. A pressure of mm. Hg was maintained in the vessel and the gallium arsenide rod was maintained at a temperature of 150 C. The zinc vapors deposited on the 2 mm. strip left free on the bottom side of the gallium arsenide rod. Thereafter such prepared gallium arsenide rod was subjected to zone melting whereby the actual doping and distribution of the doping substance in the gallium arsenide rod was effected.

Similarly boron can be applied to silicon using a piece of boron or a boron silicon mixture as vapor source and highly heating the surface of such boron or boron silicon mixture by high frequency electric heating, or electron or ion bombardment. More or less doping substance evaporates and deposits depending upon the temperature of the doping body and surrounding pressure.

All other substances named above can be doped in an analogous manner. This mode of operation above all provides the advantage that the quantity of doping substance can be controlled during its application to the body to be doped, for example, optically by colors of thin platelets or by known methods of control. The vapor deposit procedure is advantageously suited when many starting bodies are to be doped all at once. In such case the individual bodies are passed over the vapor sources and thereby receive a very exactly metered amount of doping substances. Elements, compounds, alloys mixtures or doped materials are suited as doping material which may be applied by deposit from vapors.

Not only rods or similarly shaped bodies are suitable as starting bodies to be doped according to the invention. It is also possible to use tubes, bands, threads or similarly shaped bodies.

The following examples will serve to illustrate several embodiments of the process according to the invention.

Example 1 A gallium arsenide rod was arranged horizontally in a vessel and the lower side thereof covered by two metal sheets so as to have only a strip 2 mm. wide parallel to the axis of the rod uncovered. Zinc was evaporated in a molybdenum boat situated below the gallium arsenide rod. A pressure of 10- mm. Hg was maintained in the vessel and the gallium arsenide rod was maintained at a temperature of 150 C. The zinc vapors deposited on the 2 mm. strip left free on the bottom side of the gallium arsenide rod. Thereafter such prepared gallium arsenide rod was subjected to zone melting whereby the actual doping and distribution of the doping substance in the gallium arsenide rod was effected.

Similarly boron can be applied to silicon using a piece of boron or a boron silicon mixture as vapor source and highly heating the surface of such boron or boron silicon mixture by high frequency electric heating, or electron or ion bombardment. More or less doping substance evaporates and deposits depending upon the temperature of the doping body and surrounding pressure.

Example 2 A zone-purified, p-conductive silicon rod of 24 mm. diameter and 300 mm. length was supposed to receive a specific electric resistance of ohm cm., p-conductive. The resistance of the rod declined from 5000 ohm cm. to 2000 ohm cm. over a length of 250 mm. in linear direction. Said rod was fastened in a frame together with a thin, boron containing silicon rod of 2 mm. diameter (i005 mm.) and 300 mm. length. The resistance of the doping rod increased from 0.700 ohm cm. to 0.726 ohm cm. over a length of 250 mm. The doping rod was attached closely to the rod to be doped in such a way that the low ohmic end was associated with the high ohmic end of the rod to be doped. This arrangement was placed into a vacuum drawing apparatus together with a 100 ohm cm. p-conductive seed crystal, and was subjected to zone melting twice from the bottom toward the top, the first time at 0.8 mm./min., the second time at 4.5 mm./min. in order to make the rod mono-crystalline. After the zone melting procedure the rod had an electric resistance of 98:2 ohm cm.

Example 3 A p-conductive silicon rod with a resistance of approximately 3'000 ohm cm. and with a diameter of 18 mm. was supposed to receive a resistance of 20 ohm cm., nconductive. A phosphorus containing doping rod of equal length and :3.2 mm. diameter (10.1 mm.) and 0.61 ohm cm. was associated closely in a parallel Way with the rod to be doped and stretched in a frame. Together with a seed crystal of 21 ohm cm. and 18.5 mm. diameter the arrangement was placed in a protective gas draw ing apparatus. A zone was drawn through the rod at 3 mm./min. Already after the first drawing the rod became mono-crystalline. The rod possessed a resistance of 22 ohm cm. at the end of the seed crystal, which declined steadily to 19 ohm cm. at the poly-crystalline end at a diameter of 18.3 mm.

Example 4 In a graphite boat of 200 mm. length and of a somehow semicircular cross-section of 10 mm. radius there was placed intrinsic conducting germanium, which was supposed to be doped to 0.1 ohm cm., n-conductive. As doping bodies were used angular germanium platelets 1 mm. thick and of 4 mm. edge length having a resistance of 1X10" ohm cm., n-conductive (containing antimony). According to calculation approximately 20 platelets had to be distributed lengthwise on the germanium rod, which was done in intervals of 10 mm. The first platelet was placed at the very end of the rod. From there, a zone was drawn through the germanium under protective gas. Afterwards the germanium possessed a resistance of 0.095 ohm cm. '-2%.

We claim:

A process for doping silicon rods in a crucibleless zone melting process which comprises associating a silicon rod containing a doping substance selected from the group consisting of boron and phosphorus with a silicon rod to be doped of a larger cross-section parallel to the longitudinal axis thereof and in close contact therewith and zone melting the associated silicon rods with the travelling molten zinc encompassing the entire cross-section of both rods in a protective gaseous atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,739,088 Pfann Mar. 20, 1956 2,794,846 Fuller June 4, 1957 2,953,529 Schultz Sept. 20, 1960 2,970,111 Hoffman et al. Jan. 31, 1961 

